1. Field of the Invention
The present invention relates to piezoelectric electroacoustic transducers such as piezoelectric sounders, piezoelectric receivers, and piezoelectric speakers.
2. Description of the Related Art
Piezoelectric electroacoustic transducers for emitting warning sounds or operating sounds have been widely used in electronic devices, consumer products, and cellular phones, for example, as piezoelectric sounders or piezoelectric receivers. Piezoelectric electroacoustic transducers incorporating a rectangular diaphragm have been proposed to achieve higher production efficiency, higher electroacoustic conversion efficiency, and size reduction.
Extremely thin diaphragms, on the order of tens to hundreds of micrometers in thickness, have recently been used for lower frequencies. The frequency characteristics of such thin diaphragms are greatly affected by the structures that support the diaphragms.
If, for example, a diaphragm is directly connected to terminals fixed to a casing using a thermosetting conductive adhesive, a stress due to the curing and contraction of the conductive adhesive causes a strain on the diaphragm. This strain results in variations in the frequency characteristics of the diaphragm. In addition, the cured conductive adhesive can disadvantageously obstruct the vibration of the diaphragm or, conversely, can be cracked by the vibration thereof because the cured adhesive has a relatively high Young's modulus.
Japanese Unexamined Patent Application Publication No. 2003-9286 proposes a piezoelectric electroacoustic transducer including a piezoelectric diaphragm, a casing having a support on an inner portion thereof to support the bottom surface of the piezoelectric diaphragm at two or four sides thereof, terminals having inner connection portions exposed near the support, a first elastic adhesive applied between the periphery of the piezoelectric diaphragm and the inner connection portions of the terminals to fix the piezoelectric diaphragm to the casing, a conductive adhesive applied between electrodes of the piezoelectric diaphragm and the inner connection portions of the terminals across the top surface of the first elastic adhesive to electrically connect the electrodes of the piezoelectric diaphragm to the inner connection portions of the terminals, and a second elastic adhesive provided to seal a gap between the periphery of the piezoelectric diaphragm and the inner portion of the casing. The first elastic adhesive is, for example, a urethane adhesive. The second elastic adhesive is a material with a lower Young's modulus than the first elastic adhesive, for example, a silicone adhesive.
In this case, the elasticity of the first elastic adhesive prevents, for example, variations in the frequency characteristics of the diaphragm which are caused by a stress due to the curing and contraction of the conductive adhesive and the cracking of the cured conductive adhesive. However, the support may restrain the piezoelectric diaphragm and obstruct the bending vibration thereof because the support supports the piezoelectric diaphragm at two or four sides thereof.
Japanese Unexamined Patent Application Publication No. 2003-23696 discloses a piezoelectric electroacoustic transducer including a piezoelectric diaphragm, a casing having supports for supporting the bottom surface of the piezoelectric diaphragm at the four corners thereof, a first elastic adhesive applied between the piezoelectric diaphragm and terminals near the supports, and a conductive adhesive applied across the first elastic adhesive to electrically connect the piezoelectric diaphragm to the terminals.
In this case, the supports have a small supporting area because they support only the corners of the piezoelectric diaphragm. This electroacoustic transducer can produce a higher sound pressure without restraining the diaphragm.
A piezoelectric electroacoustic transducer having supports for supporting a piezoelectric diaphragm at the corners thereof can thus produce a higher sound pressure. A smaller diaphragm-supporting area is required for further size reduction and still higher sound pressures, and a smaller diaphragm thickness is required for lower frequencies. A thinner diaphragm, however, bends more easily, and an impact, for example, can cause a large curvature of the diaphragm if the supporting area is reduced. A large curvature of the diaphragm causes a large amplitude of vibration thereof in the vicinity of the conductive adhesive, and accordingly, an excessive stress acts on the conductive adhesive. The excessive stress can disadvantageously contribute to the cracking of the conductive adhesive, thus degrading the connection reliability.
FIGS. 14A and 14B illustrate sectional views of a support supporting a piezoelectric diaphragm according to the known art.
In FIG. 14A, a support 32 supports a corner of a diaphragm 30. An elastic adhesive 34 is applied between the diaphragm 30 and a terminal 33 that is inserted in a case 33. The elastic adhesive 34 is, for example, a urethane adhesive. A conductive adhesive 35 is applied across the elastic adhesive 34 to electrically connect an electrode of the diaphragm 30 to the terminal 33.
In this support structure, the diaphragm 30 bends downward with the support 32 acting as a fulcrum if an impact, for example, applies a downward acceleration G to the diaphragm 30, as shown in FIG. 14B. The downward bending imposes a tensile stress on the conductive adhesive 35 and causes a crack.
Japanese Unexamined Utility Model Registration Application Publication No. 7-16500 discloses a piezoelectric sounder including a unimorph piezoelectric diaphragm and a case having curvature-preventing columns extending from the bottom surface thereof. The curvature-preventing columns limit the curvature of the piezoelectric diaphragm if an impact, for example, applies an external force exceeding the bending strength of the diaphragm. However, the curvature-preventing columns are intended to prevent the cracking of the piezoelectric diaphragm itself and the delamination of a ceramic plate from a metal plate, and no consideration is given to the cracking of a conductive adhesive as described above.